Field of the Invention
[0001] The present invention relates to the field of telecommunication and more particularly
to a method and corresponding apparatus for switching in a telecommunications equipment
between an active and a redundant signal in a hitless manner, i.e., without corrupting
data transmitted by the active and redundant signal.
Background of the Invention
[0002] In telecommunications networks, reliability and failure resistance is a primary issue
and telecommunications equipment shall ensure continued operation even in the case
any equipment component fails. Therefore, network elements are provided with equipment
protection, which means that core components are provided twice so that a redundant
system component can take over operation of a failed system component.
[0003] Typical network elements of a transmission network such as crossconnects and add/drop
multiplexers have a switching matrix for randomly connecting signals from any input
to any output. This switching matrix (referred to as copy A) is typically protected
by a redundant second matrix (referred to as copy B).
[0004] The principle of equipment protection is that all signals are duplicated and distributed
to the active and the redundant standby equipment component, which both perform the
identical operation on the signal. On the output side, the two signals are combined
again by selecting the better of the two for further processing. So, if the active
component fails, the signal processed by the standby component is selected and the
standby component thus becomes active.
[0005] However, loss of signal detection may take some time, during which a bit error burst
is transmitted before the protection switch is activated. This situation results in
a "hit" in the output signal and it is thus desirable to perform hitless protection
switching, i.e., to switch from active to standby signal before a disruption in the
active signal may reach the output.
[0006] Known hitless protection switch systems have a large buffer for synchronization of
the data signals. This buffer allows to base the loss of signal detection on a loss
of frame event. Known failure detectors therefore need a long time (expressed in bit
periods) to detect a failure condition and switch from active to standby signal. Moreover,
a large buffer impacts the signal delay performance during normal operation and involves
higher equipment cost and higher power dissipation.
[0007] It is thus an object of the present invention to provide a hitless equipment protection
system and method that does not require large buffers.
Summary of the Invention
[0008] These and other objects that appear below are achieved by detecting signal transition
in both copies of a protected signal and by switching from the active copy to the
standby copy if the former does not show any signal transition any more but the latter
does.
[0009] In particular, a network element according to the present invention has first and
second redundant signal paths for first and second redundant signals; a selector for
selecting either of the two redundant signals as active; and first and second transition
monitors coupled to the first and second signal paths, respectively, for monitoring
the first and second signals for bit level transitions. The selector is controlled
by the transition monitors to alter selection in the case that the selected signal
does not contain bit level transitions while the non-selected signal does.
[0010] The invention allows immediate detection of a failure condition and to switch over
in a hitless manner at the time of detection of a failure. This greatly facilitates
maintenance because there is no need anymore to initiate the protection switch earlier
in time in order to avoid a traffic hit.
Brief Description of the Drawings
[0011] Preferred embodiments of the present invention will now be described with reference
to the accompanying drawings in which
figure 1 shows a block diagram of a network element;
figure 2 shows a block diagram of a second embodiment of a network element according
to the invention; and
figure 3 shows a state diagram for the selector according to the invention.
Detailed Description of the Invention
[0012] Network elements typically have equipment protection installed, which means that
certain core processing units such as for example a switch matrix is provided in duplicate
so that the second redundant processing units can take over operation if the first
active processing unit fails. Internally, the network element has thus two redundant
signal paths, one leading over active matrix copy A and one over redundant matrix
copy B. The signal that is protected that way, is copied to either of the two signal
paths, so that the paths carry identical signals. At the output side, however, the
better of the two signals is selected for output. As explained above, in today's network
elements the switch-over from active to redundant signal path is either not hitless
or requires a large frame buffer.
[0013] The components in a network element that are involved in the selection according
to the invention are shown in a first embodiment in figure 1. The network element
has an equipment clock CL, two redundant signal paths P1, P2 and a switch SW selecting
one of the two for output O. Each signal path is fed to an amplifier A1, A2, respectively
clocked by clock CL, which leads to a delay line D1, D2. The amplified signal is also
fed to a transition monitor T1, T2. The output of the transition monitor is coupled
to a selector C, which controls the operation of the switch SW.
[0014] The working principle of the circuit according to the invention is simple: Transition
monitors T1, T2 are provided on each data signal copy P1, P2. All the circuits are
synchronously clocked by the system clock CL and the data signal copies are frame
aligned. The copy which does not show any transition on a sliding block of N bits
is assumed to be faulty. Between the monitor T1, T2 and the hitless switch SW there
are delay elements D1, D2 in the data signal stream with transmission delay equivalent
to N bits at least. N bit period is the time needed to detect a loss of signal condition
and operate the hitless switch.
[0015] The delay line can be implemented by a simple shift register of N bit depth clocked
by the system clock or by any other delaying component, including for example a FIFO
(first-in-first-out buffer) or a cable or wave guide delay line with appropriate length
so that the total propagation delay equals N bits. The amplifiers A1, A2 are optional
and may be discarded if not needed.
[0016] The transition monitors are clocked by system clock CL and detect bit level transitions,
i.e., whether the signal level changes from high to low or vice versa from one clock
pulse to the next. If no such bit level transitions occur in the input signal, the
latter is assumed to be faulty. If at the same time, the other signal shows transitions,
the selector C triggers a switch-over from the alleged faulty signal to the one still
showing transitions, provided that the former was selected before.
[0017] The signal copies are selected according to table 1 below:
Table 1:
Selection criteria for copy A/B |
History |
copy A |
copy B |
select copy |
A |
OK |
OK |
A |
B |
OK |
OK |
B |
A |
NOK |
OK |
B |
B |
NOK |
OK |
B |
A |
OK |
NOK |
A |
B |
OK |
NOK |
A |
A |
NOK |
NOK |
A |
B |
NOK |
NOK |
B |
[0018] If for example signal copy A was selected and signal copies A and B show transitions
and are thus assumed to be all right, then signal copy A remains selected (line 1).
If signal copy A was selected and A is found to be faulty (NOK) but B appears to be
all right, selection will be reverted from signal A to signal B (line 3). However,
if both signal copies are found to be faulty, then no switch-over is initiated (lines
7 and 8).
[0019] If the data signal consists of a long sequence of identical digits then the both
transition monitors qualify either copy as being NOK, but according to the table above
no copy switch is initiated. However, if in such case copy A fails there may be a
transition to the low level which would initiate a switch over to the failed copy.
In order to avoid this, the signal state can be used in this particular situation
as an additional criterion. Thus if both signals do not shows transients, a copy is
OK if the signal state is "active" otherwise it is NOK.
[0020] In the first embodiment, however, a problem may occur if the failed copy of the data
signal toggles between two signal states. In this case, the proposed mechanism will
fail. Thus, in a second improved embodiment, the selection is made more robust against
such failure by allowing transition based selection only if the data signal itself
is found to be valid, e.g., by presence of a frame pattern. A second monitor is therefore
added, which searches for significant signal parts in the signals and enables transition-based
selection only, if such significant signal part is found. This second criterion is
given the higher priority.
[0021] The circuit according to the second embodiment is shown in figure 2. It contains
all circuit components from figure 1 and in addition first and second frame monitors
F1, F2 linked to the first and second signal paths P1, P2, respectively. The two frame
monitors lead to a first selector S1, which controls the switch SW with first priority,
i.e., only if a signal is found to be valid, it can be selected for output. The transition
monitors T1, T2 lead to a second selector S2, which controls the switch SW in the
way described above, but with the above restriction that switch-over is only enabled
if the selected signal is found to be valid by the corresponding frame monitor. If
the first selector finds one signal valid and the other invalid, it switched to the
valid one, irrespective what the second selector results. The second selector S2 has
thus a second priority lower than the first priority of the first selector S1.
[0022] The selection criteria for a priority 1 selection based on significant signal parts
(e. g. framing) is shown in table 2 below.
Table 2:
Priority 1 selection criteria using frame monitor |
History |
copy A |
copy B |
selection |
A |
OK |
OK |
Allow for prio 2 selection |
B |
OK |
OK |
Allow for prio 2 selection |
A |
NOK |
OK |
B |
B |
NOK |
OK |
B |
A |
OK |
NOK |
A |
B |
OK |
NOK |
A |
A |
NOK |
NOK |
A |
B |
NOK |
NOK |
B |
[0023] Only if both signal copies appear to be valid by the frame monitor, priority 2 selection
based on the transition monitors is enabled according to table 3 below.
Table 3:
Priority 2 selection criteria using transition monitor |
History |
copy A |
copy B |
select copy |
A |
OK |
OK |
A |
B |
OK |
OK |
B |
A |
NOK |
OK |
B |
B |
NOK |
OK |
B |
A |
OK |
NOK |
A |
B |
OK |
NOK |
A |
A |
NOK |
NOK |
A |
B |
NOK |
NOK |
B |
[0024] However, toggling of a failed signal can normally also be avoided by the use of suitable
pull-down or pull-up circuits at preceding data signal driver output or receiver input
circuits.
[0025] When one data signal breaks, a hit in the failed signal may occur, which causes an
extra transition, e.g. from high level to low level. Such a hit should not cause the
system to switch to the failed signal. ln another preferred improvement of the invention,
a timer is thus provided and switch-over is only initiated if the failure condition
in the currently selected signal persists after a timeout. This improvement makes
the selection also robust against some signal toggling before the loss condition is
reached.
[0026] Figure 3 shows a state diagram of this third embodiment. After the start of the system,
it is in a monitoring state ST1. If the monitor detects as condition C1 an inconsistency
between copies A and B data signals, the second state ST2 is reached after M bits.
In the state ST2, the system is loaded to switch immediately to that signal copy which
shows the first bit transition (i.e., second condition C2). Switch over is shown as
signal state ST3. After switch-over, the system goes back to the state ST1 again (condition
C3). If not switch-over occurs for a predefined time interval (i.e., condition C4),
the system goes back to the state ST1, as well. The timeout period may be selected
according to known signal characteristics, e.g. the expected bit transitions associated
with the frame alignment word, or according to signal statistics, e.g., the largest
possible number of CID (Consecutive Identical Digits) to be supported.
[0027] While the selection mechanism has been described with respect to equipment protection
switching, it should be noted that it is equally applicable to other type of protection,
i.e., path protection or section protection.
1. A network element comprising first (P1) and second (P2) redundant signal paths carrying
first and second redundant signals, respectively; a selector (SW) for selecting either
of the two redundant signals as active; and first (T1) and second (T1) transition
monitors coupled to the first and second signal paths, respectively, for monitoring
said first and second signals for bit level transitions; wherein said selector (SW)
is controlled by the transition monitors (T1, T2) to alter selection in the event
that the selected signal does not contain bit level transitions while the non-selected
signal does.
2. A network element according to claim 1, comprising first and second delay elements
(D1, D2) of substantially N bit depth coupled to said first and second signal paths
(P1, P2), respectively, wherein said selector (SW) is controlled to alter selection
when the selected signal does not contain bit level transitions for a bit sequence
of N bits while the non-selected signal does contains bit level transitions in the
same interval.
3. A network element according to claim 1, further comprising first (F1) and second (F2)
frame monitors for monitoring said first and second signals, respectively, for the
presence of predefined signal patterns, wherein said signals appear to be valid if
said predefined bit pattern is detected and wherein switch-over from one to the other
signal according to detection of bit level transition is enabled only, if both signal
copies appear to be valid.
4. A network element according to claim 1, further comprising a timer, wherein switch-over
from one to the other signal according to detection of bit level transition is enabled
only if after lapse of said timer the condition persists that the selected signal
does not contain bit level transitions.
5. A network element according to claim 1, further comprising pull-up or pull-down circuits
for pulling a failed signal to a predefined level.
6. A selection circuit for a network element comprising first (P1) and second (P2) redundant
signal paths carrying first and second redundant signals, respectively, said circuit
comprising a selector (SW) for selecting either of the two redundant signals as active
and first (T1) and second (T2) transition monitors coupled to the first and second
signal paths (P1, P2), respectively, for monitoring said first and second signals
for bit level transitions; wherein said selector (SW) is controlled by the transition
monitors (T1, T2) to alter selection in the event that the selected signal does not
contain bit level transitions while the non-selected signal does.
7. A method of controlling selection of either of first and second signals from first
(P1) and second (P2) redundant signal paths in a network element, said method comprising
the steps of
- selecting either of the first and second redundant signals as active signal;
- monitoring said first and second signals for bit level transitions; and
- altering selection in the case that the selected signal does not contain bit level
transitions while the non-selected signal does.